1. Field of the Invention
The invention relates to a cold cathode having a micro-sized structure and formed by means of thin film making technique, a method of fabricating such a cold cathode, and a light emitting apparatus including such a cold cathode, in particular, a light emitting apparatus suitable for a back light device to be used for a liquid crystal display, and also for a large size display.
2. Description of the Related Art
There has been suggested a field emission cold cathode by C. A. Spindt, "A Thin-Film Field-Emission Cathode", Journal of Applied Physics, Vol. 39, No. 7, pp. 3504-3505, 1968. The suggested field emission cold cathode has a plurality of unitary cold cathodes arranged in an array, each unitary cold cathode comprising a conically shaped emitter, and a control electrode or a gate electrode disposed in the vicinity of the emitter and having functions of extracting a current from an emitter and controlling an amount of the thus generated current. FIG. 1A illustrates the suggested field emission cold cathode 108, and FIGS. 1B and 1C illustrate cross-sections of a unitary cold cathode 107 constituting the field emission cold cathode 108 together with other unitary cold cathodes.
The field emission cold cathode 108 includes a silicon substrate 101, a insulating layer 102 made of silicon dioxide and formed on the silicon substrate 101, and a control electrode 103 formed on the insulating layer 102. A part of the insulating layer 102 and the control electrode 103 is removed to thereby form a cavity 109, in which an emitter 104 having a sharpened tip end is formed on the silicon substrate 101. The emitter 104, the control electrode 103 and the cavity 109 cooperate with each other to thereby form a unitary cold cathode 107. A plurality of unitary cold cathodes 107 are arranged in an array to thereby form the field emission cold cathode 108 having a planar electron emission area.
The silicon substrate 101 and the emitter 104 are in electrical contact with each other. A voltage of about 50 V is applied across the emitter 104 and the control electrode or gate electrode 103. A thickness of the insulating layer 102 and a diameter of an opening 103a formed with the gate electrode 103 are both quite small. Specifically, the insulating layer 102 has a thickness of about 1 .mu.m, and the opening 103a of the gate electrode 103 has a diameter of about 1 .mu.m. In addition, a tip end of the emitter has a diameter of about 10 nm, and hence is quite sharpened. Thus, by applying a voltage of about 50 V across the emitter 104 and the gate electrode 103, an intensive electric field is applied to a tip end of the emitter 104. When the thus generated electric field has an intensity in the range of 2.times.10.sup.7 V/cm to 5.times.10.sup.7 V/cm, a mass of electrons are emitted through a tip end of the emitter 104. By arranging a plurality of the above mentioned unitary cold cathodes 107 on the silicon substrate 101 in an array, there can be formed a planar cathode which is capable of generating a large amount of current. In addition, an arrangement of the above mentioned unitary cold cathodes in high density by utilizing micro-size-processing technology makes it possible to increase a current density five to ten times greater or more in comparison with a prior hot cathode.
The above mentioned Spindt type cold cathode has merits that it provides a higher cold cathode current density than a hot cathode, and that velocity dispersion of emitted electrons is smaller than a hot cathode. In addition, the Spindt type cold cathode emits smaller current noise in comparison with a single field emission cold cathode, and can operate even with a small voltage, specifically a voltage in the range of about 10 V to tens of V. Furthermore, the Spindt type cold cathode can operate even in relatively poor vacuum, specifically, in the range of about 10.sup.-5 Pa or greater.
There has been suggested another cold cathode in order to control focusing of electron beams emitted through a tip end of the emitter 104 by W. D. Kesling et al., "P-42: Field-Emission Display Resolution", SID 93 DIGEST, pp. 599-602, 1993. The suggested cold cathode includes, as illustrated in FIG. 1C, a silicon substrate 101, a silicon dioxide insulating layer 102 formed on the silicon substrate 101, a gate electrode 103 formed on the insulating layer 102, a second insulating layer 105 formed on the gate electrode 103, and a focusing electrode 106 formed on the second insulating layer 105. The insulating layer 102 is formed with a cavity 109a in which an emitter electrode 104 is disposed, and the second insulating layer 105 is formed with a cavity 109b. The gate electrode 103 is formed with an opening 103a, and the focusing electrode 106 is formed with an opening 106a. The cavities 109a and 109b and the openings 103a and 106a are in communication with one another.
As illustrated in FIG. 2, there has been suggested still another cold cathode including a silicon substrate 101, an insulating layer 102 formed on the silicon substrate 101, a control electrode 105 formed on the insulating layer 102 around an emitter electrode 104 which is disposed within a cavity 109 formed with the insulating layer 102, and a ring-shaped focusing electrode 111 formed on the insulating layer 102 around the control electrode 105 (K. Yokoo et al., "Technological Breakthrough in Development of Field Emitter Display", Proceedings of the First International Display Workshops, pp. 19-22, 1994). The ring-shaped focusing electrode 111 controls trajectory of electrons emitted from the emitter 104.
There have been also suggested various apparatuses for emitting electrons from a cold cathode such as those mentioned above to a fluorescent material to thereby make the fluorescent material luminous for displaying various information. For instance, such apparatuses are a planar display apparatus or a simple light emitting apparatus. A planar display apparatus is used as an output device of a computer or a display screen of a television set. By combining a plurality of light emitting apparatuses with one another, the combined light emitting apparatuses can be used as a large display screen for displaying letters information and/or television signals. Since those planar display apparatus and light emitting apparatus do not need to have a heater for heating a cold cathode and can use fluorescent material having a relatively high light emitting efficiency, those apparatuses are characterized by a high operation efficiency. With development of an information-oriented society, a demand for a display apparatus which is one of information output machines is predicted to increase, and power consumption in a display apparatus is required to be reduced. A display apparatus using a cold cathode therein is in line with such requirement.
U.S. Pat. No. 4,818,914 issued to Brodie has suggested a light emitting apparatus using a field emission type cold cathode. As illustrated in FIG. 3, the suggested light emitting apparatus includes an evacuated envelope 10 having an annular wall 12 of insulating material which is closed at one end by a light-transmitting member 14 and at the other end by a substrate 16. A highly conductive doped silicon layer 20 is deposited on the substrate 16 upon which layer an array of individual cathodes 22 is formed. A unitary accelerator electrode 28 is formed on a dielectric film 24. A unitary anode electrode 30 is deposited on an inner surface of the light-transmitting member 24, and a phosphor layer 32 is deposited on the anode electrode 30. A power source 36 applies a voltage across the silicon layer 20 and the anode electrode 30 to thereby cause electrons 42 to emit from the cathodes 22.
Japanese Unexamined Patent Publication No. 4-286852 has suggested a light emitting apparatus including a field emission cold cathode as illustrated in FIG. 4. The illustrated light emitting apparatus includes an electron beam source 1, a fluorescent material layer 2, a transparent electrode 3, and a deflecting electrode 4. Japanese Unexamined Patent Publication No. 4-286853 has suggested another light emitting apparatus including a field emission cold cathode as illustrated in FIG. 5. The illustrated light emitting apparatus includes an electron beam source 1, a collector plate 2 formed centrally with an opening window 5, a fluorescent material layer 3 formed on the collector plate 2, and a transparent electrode 4. Japanese Unexamined Patent Publication No. 4-286854 has suggested still another light emitting apparatus including a field emission cold cathode as illustrated in FIG. 6. The illustrated light emitting apparatus includes an electron beam source 1, a reflection plate 2 formed centrally with an opening 5, a transparent electrode 4, and a fluorescent material layer 3 formed at a surface of the transparent electrode 4.
The above mentioned United States Patent and Japanese Unexamined Patent Publications have suggested a method of spreading electrons in a greater emission angle which electrons are emitted from an electron emitting source having a smaller electron emitting area than an area of a fluorescent layer.
For another instance, there has been suggested an application of a panel constituted of thin-film-edge field emission cold cathodes to a backlight of a liquid crystal display device by A. I. Akinwande et al., "Thin-Film-Edge Emitter Vacuum Microelectronics Devices for Lamp/Backlight Applications", IVMC 95, pp. 418-422, 1995.
Japanese Unexamined Patent Publication No. 53-121454 has suggested a cold cathode including a substrate, emitters formed on the substrate, a first insulating layer formed on the substrate and formed with cavities in which the emitters are disposed, a gate electrode formed on the first insulating layer and formed with openings above the emitters, a second insulating layer formed on the gate electrode and formed with cavities above the emitters, and a focusing electrode formed on the second insulating layer and formed with openings above the emitters. The openings of the focusing electrode may be offset so that the focusing electrode is disposed just above the emitters in order to prevent direct ion bombardment to summits of the emitters.
Japanese Unexamined Patent Publication No. 4-133241 has suggested a cold cathode including an emitter projecting above a gate electrode.
The light emitting apparatus suggested in U.S. Pat. No. 4,818,914, illustrated in FIG. 3, needs to have a cold cathode 18 having almost the same area as that of the phosphor layer 32 for emitting electrons therethrough, resulting in necessity of a large area cold cathode. In addition, since the evacuated envelope 10 is cylindrical in shape, the cold cathode 18 is also required to be cylindrical in shape. In general, when a cold cathode is to be fabricated, a plurality of unitary cold cathodes are simultaneously fabricated by means of a thin film fabrication apparatus, similarly to fabrication of a semiconductor device. Hence, a unitary cold cathode is required to be as small as possible in size and rectangular in shape for conforming to semiconductor device fabrication process and also for reducing fabrication costs. However, the cold cathode used in the light emitting apparatus illustrated in FIG. 3 does not meet such conditions.
The light emitting apparatuses illustrated in FIGS. 4 and 5 attempts to make electron beams emitted from the cold cathode 1 diverge to thereby make luminescent the fluorescent screen 2 having a larger area than that of the cold cathode 1. The illustrated light emitting apparatuses both have the electrode 4 for diverging electron beams, disposed in an evacuated enclosure, and are in operation by applying a voltage to the electrode 4.
In the light emitting apparatus illustrated in FIG. 6, a light backwardly reflected from the fluorescent material layer 3 is forwardly reflected by the reflection plate 2 disposed in an evacuated enclosure.
In all of the light emitting apparatuses illustrated in FIGS. 4 to 6, electron beams are enlarged or a light is reflected and enlarged both by means of an electrode or an optical device. Thus, those light emitting apparatuses have problems that they are complicated in structure, and they have to have a number of parts.
It is necessary to prepare an electron emission source having a large area in order to construct a large luminescent area like a backlight in a thin panel structure. In such a electron emission source, in order to prevent the entire electron emission source from stopping in operation because of discharge breakdown and/or shortcircuit of a part thereof, it is necessary to divide an electron emission area into a plurality of sections and provide a fuse with each sections for removing a section destroyed by discharge breakdown or a shortcircuited section. When such a section is removed, electrons are not emitted from a removed section of the electron emission area, resulting in nonuniformity in luminance in a luminescent screen.